Mountain bike frames are designed for rough treatment. They have built-in features that enable them to withstand years of punishment. Other features include the use of exotic materials such as carbon fiber. Some frame geometry can be beneficial while other changes can be risky. Evaluate a few factors before purchasing a mountain bike.

Strength

Mountain bike frames must be strong. The incredible beating they take while you are riding is one thing, but mountain bikes crash. They often go bouncing down the hillside without riders. It's vital that the frame can take it. When evaluating a mountain bike frame, look at the welds on metal frames. They should look smooth and even with no irregularities. The frame shouldn't have too many angles that stray from traditional bike geometry. If the bike looks like some scientific experiment gone horribly wrong, it's probably not very strong. The fewer the joints, the stronger the frame will be. Carbon frames should look thick, with smooth transitions where the frame is joined. Narrow or thin frames should be avoided. Compare a mountain bike frame to a road bike frame. It should be much thicker.

Dropouts

Dropouts are where the axle of the back wheel slides into the frame. These small hook-shaped cut-outs in the frame support all of your weight, plus accumulated weight when you and your bike pull a positive G-force coming into a dip or compression. If dropouts are not strong, the back wheel can break off. Frame dropouts should look substantial. They shouldn't look bent or worn. If you can see shiny metal when you remove the rear tire, it could mean the the dropout is worn badly or it is compromised. Some mountain bikes have removable dropouts. This is a good idea. If they get worn or bent, you can have them replaced.

Bottom Bracket

The bottom bracket is where the pedal axles pass through the bike frame. It's the round cylinder where the sealed bearing is housed. If the bottom bracket is not sufficiently thick, when you crank hard on the pedals it will bend. Also known as bottom bracket flex, this is fairly common in bikes, but it shouldn't be. To test the bottom bracket, ride the bike around a steep corner while pedaling hard. If the bottom bracket is too thin or it is worn, you might feel a click or thump in the pedals or even see the gear bend to the side as you crank hard on the pedal. This frame failure will eventually result in bad bearings, causing the crank to fail.

Carbon vs. Aluminum

The majority of mountain bike frames are made with aluminum or chromoly steel. Even though most contemporary road bikes use carbon, the beating that mountain bikes absorb makes steel a better choice. Carbon is strong enough, but if it gets even a small crack it can shatter, although that scenario is unlikely. Aluminum is less likely to fail even if it gets a small crack. But if you spend enough money on a high-end carbon or aluminum bike frame, it should last a lifetime, and most manufacturers guarantee a bike frame for life. Check for this before purchasing. Carbon frames are more comfortable. Carbon fiber frames absorb trail and road vibration better than aluminum or steel. Steel frame bikes are stiff and transfer road vibrations to the rider more than carbon. This results in a rougher ride.

Size

Mountain bike frames come in different sizes, although it's not as critical as road bike frame sizes. Mountain bikes are forgiving. Some manufacturers use only three size designations: small, medium and large. Other manufactures size their bike frames according to a measurement from the center of the crank to the top of the frame where the seat post exits the frame. These measurements are typically 16, 18 and 19 inches. An average-sized person typically will require at least an 18-inch frame. Mountain bike sizing is not an exacting science. The key to finding the right size is getting on one and riding it. You should feel comfortably stretched out, with about a 5-degree bend in your arms, with a similar 5-degree bend in your legs at the bottom of the pedal stroke.

References

About the Author

Specializing in hardwood furniture, trim carpentry, cabinets, home improvement and architectural millwork, Wade Shaddy has worked in homebuilding since 1972. Shaddy has also worked as a newspaper reporter and writer, and as a contributing writer for Bicycling Magazine. Shaddy began publishing in various magazines in 1992, and published a novel, “Dark Canyon,” in 2008.